This invention relates to novel porous mullite articles, the manufacture thereof and to the use of the porous mullite articles as contact materials and catalyst supports. The invention relates also to novel procedures for preparing mullite materials, including shaped mullite materials, such as small spheres (microspheres). The procedure involves calcining clay, which may be in the form of shaped preformed bodies, to a temperature at which the clay undergoes an exothermic reaction and below which substantial mullite is formed, and thereafter removing silica from the bodies by leaching with a strong base and then calcining the bodies at a temperature at which the leached clay undergoes a reaction to form mullite.
Kaolin clays are hydrated aluminum silicates of the approximate formula Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O. It is well known that when kaolin clay is heated in air that a first transition occurs at about 550.degree. C. associated with an endothermic dehydroxylation reaction. The resultant material is generally referred to as metakaolin. Metakaolin persists until the material is heated to about 975.degree. C. and begins to undergo an exothermic reaction. This material is frequently described as kaolin which has undergone the characteristic exothermic reaction. Some authorities refer to the material as a defect aluminum-silicon spinel or as a gamma alumina phase. See Donald W. Breck, ZEOLITE MOLECULAR SIEVES, published by John Wiley & Sons, 1974, pages 314-315. On further heating to about 1050.degree. C., mullite is formed. The mullitization reaction that takes place when kaolin clay is utilized as the sole source of silica and alumina may be represented by the following equation where the approximate chemical formula for kaolin (without the water of hydration) is given as Al.sub.2 O.sub.3.2SiO.sub.2, and the formula for mullite is 3Al.sub.2 O.sub.3.2SiO.sub.2 : EQU 3(Al.sub.2 O.sub.3.2SiO.sub.2).fwdarw.3Al.sub.2 O.sub.3.2SiO.sub.2 +4SiO.sub.2.
The term represented by 4SiO.sub.2 is the free silica generated as a result of the conversion to mullite. The free silica can be amorphous or crystalline, depending on calcination temperature and time. A high purity kaolin clay can theoretically be converted into about 64% mullite on a weight basis.
Mullite is widely used in ceramic applications such as in the manufacture of refractory grains. For these applications, dense impervious products are needed and porosity is undesirable. See, for example, U.S. Pat. No. 3,462,505.
It is also well known that the reactivity of kaolin clay changes as it undergoes these thermal transitions. See the Breck publication supra at page 315. In fact, the reactivity of both silica and alumina in metakaolin with sodium hydroxide solution is utilized in the production of the crystalline zeolitic molecular sieve known as zeolite A. In such a process all or virtually all of the silica and alumina react to form crystals of Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.XH.sub.2 O. It is also known that when kaolin undergoes the exothermic reaction at 975.degree. C. silica can be leached readily with sodium hydroxide solution with minimal removal of alumina. Reference is made to U.S. Pat. No. 2,939,764 to Schoenfelder et al. The '764 patent contains figures demonstrating the effect of calcining kaolin clay at various temperature levels on the solubility of alumina and silica in sodium hydroxide solutions of concentrations ranging from 5% to 40%. The figures indicate that up to about 80% by weight of the silica content of high purity kaolin can be leached with strong caustic solutions when the kaolin has previously been calcined at about 1000.degree. C. The teachings of U.S. Pat. No. 2,939,764 are incorporated herein by cross-reference thereto.
The reaction of kaolin clay calcined to undergo the exotherm with sodium hydroxide solution can also be utilized in the manufacture of catalyst composites containing synthetic crystalline faujasite and a silica-depleted porous residue of calcined clay. Preferably, a small amount of metakaolin is incorporated into the reaction mixture. It is also known that during such reaction, the original sodium hydroxide solution is converted into a solution that comprises sodium silicate, reflecting silica removal from the calcined clay and the resulting development of a porous nonzeolitic component.
When kaolin clay is thermally transformed into mullite and silica, it is known that the silica can be extracted from the mullite with a strong base. See for example, U.S. Pat. No. 2,536,122 and Japanese Patent Application No. 69 84,626 (CA81(10)53673a). It is my understanding that removal of the free silica in this fashion (or by reaction with sources of Al.sub.2 O.sub.3) to form additional mullite is practiced to improve the refractoriness of the resultant solid. In this regard, it is noted that the Al.sub.2 O.sub.3 --SiO.sub.2 phase diagram (Phase Diagrams for Ceramists, Amer. Cer. Soc. Ed., 1964, Diagrams 313-314) shows that pure mullite does not melt until about 1850.degree. C.; however, in the presence of free silica, melting begins at only about 1600.degree. C. Therefore, by eliminating free silica, the refractoriness of millite is improved to an extent such that the melting point is about 250.degree. C. higher. With regard to the prior practice of removing silica from mullite produced by calcining clay, U.S. Pat. No. 2,536,122 describes grinding the clay after calcination and before the extraction step. It is reasonable to conclude that when the resulting leached mullite grains are formed into refractory articles, porosity in the finished articles is reduced by addition of binders and also by sintering procedures conventionally used in manufacturing mullite products. To the best of my knowledge and belief, there has been no acknowledgment or appreciation of the fact that mullite purification by removal of silica from mullitized kaolin wound change the pore structure.
Furthermore, the maximum amount of silica that can be extracted from kaolin calcined to mullite phase is only about 36% by weight, limiting the molar SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the resultant solid product to about 3/2 and the resultant increase in porosity to about 0.25 cc/g.